Anticandidal activity of Inula helenium root essential oil: Synergistic potential, anti-virulence efficacy and mechanism of action

•Inula helenium essential oil was evaluated for anticandidal efficacy.•The oil showed high synergism with nystatin as well as potent anti-biofilm action.•All tested virulence factors of the strains were highly inhibited in contact with the oil.•The sorbitol and cholesterol experiments showed cell membrane-targeted action.•Results point to high applicative potential of this oil for the treatment of candidiasis. Motivated by the ethnopharmacological application of Inula helenium, in the present study, the anticandidal potential of hydrodistilled root I. helenium essential oil was investigated by a microdilution method and the obtained minimal inhibitory concentrations (MICs) were further used to explore its synergistic potential. Additionally, the mode of action (sorbitol and cholesterol assays) and anti-virulence effects (anti-biofilm, germ-tube reducing and phospholipase-inhibitory activities) were also studied. GC and GC/MS analyses showed that the isolated essential oil contained alantolactone and isoalantolactone as the dominant compounds (65.8 % and 25.5 %, respectively). The obtained MICs (0.009-0.312 mg/ml) varied for the strains generally demonstrating a medium-to-high anticandidal action. More importantly, the oil exhibited a high synergistic effect in combination with the antifungal agent nystatin. Experiments on the mode of action demonstrated that the oil affected the cell-membrane function. Considering virulence factors, the oil showed an antibiofilm activity, as well as a very high germ-tube reducing potential. The results demonstrated a complete inhibition of the enzyme phospholipase in the presence of the tested I. helenium root essential oil. Based on the present results, I. helenium oil could be considered a natural agent that deserves to be further explored as a crop for the treatment of candididases.